Basement stairs in a new house: what are the code limits on riser height, tread depth, headroom, and how consistent the steps have to be?

IRC Residential Stair Code (2018/2021):

• Riser Height: Maximum 7 inches.
• Tread Depth: Minimum 10 inches (measured from nosing edge to back of tread).
• Headroom: Minimum 6 feet 8 inches (measured vertically from the nosing line of each tread).
• Consistency: No single riser shall exceed the smallest riser by more than 3/8 inch. No single tread depth shall be shorter than the longest tread by more than 3/8 inch.

Additional Notes:

• Handrails required on at least one side for stairs with 3+ risers.
• Nosings must be rounded or have a max 1¼" radius on the front edge.
• Landings: Minimum width = stair width, with 36" min clear length at landings.

These are prescriptive requirements; deviations require engineering approval. If your local jurisdiction has adopted newer codes or amendments, those may apply. Always check with your local building department before cutting stringers.

2021/2024 IRC (R311.7) – Residential Basement Stairs

| Dimension | Limit | Code Ref | |-----------|-------|----------| | Max Riser Height | 7¾ in. | R311.7.5.1 | | Min Tread Depth | 10 in. (measured nosing-to-nosing) | R311.7.5.2 | | Nosing | ¾–1¼ in. required on solid risers if tread < 11 in. | R311.7.5.3 | | Headroom | 6 ft 8 in. min., measured vertically from nosing line | R311.7.2 | | Uniformity | Max ⅜ in. variation between tallest/shortest riser or deepest/shallowest tread within any flight | R311.7.5.1 / .5.2 |

Practical notes

• Measure risers finish-floor to finish-floor; account for subfloor, underlayment, and flooring thickness on both levels before cutting stringers.
• Check headroom at the tightest point (usually where the stair passes under a floor joist or beam). If < 6'-8", you need a larger opening or a winder/landing redesign.
• The ⅜-in. tolerance is total accumulated difference, not per step. Layout stringers on a single template; gang-cut or CNC to hold tolerance.
• Open risers allowed only if a 4-in. sphere cannot pass (R311.7.5.1). Most basements use closed risers anyway.
• Handrail: 34–38 in. above nosing, continuous full flight, graspable profile (R311.7.8).

Common trap: Builder frames to 7-7/8" risers "to make the math work." Fail. Max is 7¾". Recalculate run or add a landing.

Verify with your local AHJ—some jurisdictions adopt older codes (e.g., 8¼" max riser, 9" min tread) or amend headroom to 7 ft.